Intermediate frequency amplifier for television purposes



July 8; 1941. K. SCHLE'SINGER 2,248,5

' INTERMEDIATE FREQUENCY AMPLIFIER FOR TELEVISION PURPOSES Original Fiied Jan. 21, 19:55

WKW v Patented July 8, 1941 I INTERMEDIATE FREQUENCY AMPLIFIER FOR TELEVISION PURPOSES Kurt Schlesinger, Paris, France, 'assignor, by 'mesne assignments, to Loewe Radio, Inc.,. a corporation of New York Original application January 21. 1935, Serial No.

2,631. Divided and this application August 8, 1939, Serial No. 288,974. In GermanyJanuary 3 Claims. (01. 179-171) This application is a division of Schlesinger Patent No. 2,173,495, issued September 19', 1939, on application No. 2,631, filed January 21, 1935.

The subject matter of the invention is a superheterodyne receiver for wireless television over frequency amplifier with chokes as coupling 'means for use in television receiving sets. r

Since in the case of televisionit is a matter of a not excessively wide frequency band amplified about the intermediate carrier wave on either side, and the relative width of band, i. e., the amount max f vmax. being the maximum image current frequency and iithe intermediate carrier frequency,

maybe narrowed down as desired by increasing in The solution would appear obvious in'the use of resonance chokes as anode resistances. The applicant has found by experiment, however, that the use of resonance for television purposes meets with obstacles in practice. If a weakly damped circuit with a building-up time1- is impulsed by a short impulse corresponding with an image point and having the inten'dedduration At, the same resonates during the time 1', or for a longer time with lessdamping. A reverberation of this nature is integrated by the following receiving rectifier. In this way there is produced in the received picture, following sharp contour lines of light variations, an undesirable veil of light of the width a=-r.o, wherein u is the rate of movement of the image point over the line. This widening is disturbing if the same has the order of magnitude-of an image point. The limit calculated from relations known per se in respect of the admissible intermediate carrier frequency when employing sharp resonances would be: fr 20 metres (or 15 megacycles) In the case, therefore, of ISO-line images the wave length of the carrier frequencies would amount to less than 30 metres megacycles), and in the case of 240-line images to half this length (20 megacycles). The numbers are still more unfavourable in the case of a large number of amplifiers. With short Waves of this nature, however, the amplification per stage is less than with long waves. r

By damping or by the detuning of resonant combinations effective amplification over a sufficiently wide-range is obtainable. Whereas for radio purposes operations are alwaysconducted on the resonance peak and the ratio between the enforced amplitude and the free one is of the order of 1:100, the same for television purposes should be selected at'least 1110-, and if possible even greater. The limit in methods of this kind is the ohmic resistance with a ratio of 1:1.

The shorter the intermediate carrier wave is selected, the higher is the degree to which the free oscillation of the resonance chokesmaybe utilized in relation to the enforced potential, as the-reverberation periods are small in proportion to the intermediate carrier Wave. There is obtained from this the somewhat paradoxical result initially that the shorter the wavelength the better the amplification which is capable of being attained: This applies, however, only up to the accomplishment of a certain limit in-the vicinityof the relation fT=5mmax1 according to the invention and already determinedin the above. The following table confirms-these statements by measurements obtained in practice:-

h (periods per second) 10 1. 5X10 2x10 -r wave length (meters) 300 200 v 150 Amplification factor in the case of amplification by ohmic resistance coupling (1' ohmic) 2 2 2 Amplification factor in the case of amplification by inductance coupling (v inductance) 5 10 20 v induct. m 2. 5 5 10 -Number of lines .1 -.z=l lines =540 k. c.

It is desirable to select the ratio L/C between the choke inductance and the unavoidable selfcapac'ities as large as possible, as in this manner there is' obtained 'control over relatively wide frequency bands. Assistance in this connection is rendered by short connection leads and also by the use of screening grid tubes, further by the use of sufiiciently spacious screening cylinders, assuming these should be necessary.

In the practical execution of an intermediate frequency amplifier of this nature, which operates according to the detuned oscillatory choke,

it is naturally found that the operating frequency is amplified less than the resonance frequency of the choke coupling. This results in disadvantages upon .the manipulation of the ap paratus, as it has been the custom in the radio art to adjust the apparatus to the point of greatest sensitiveness. There may, however, even occur beyond this a self-oscillation of the amplifier on the resonance wave, or it may be necessary for the designer to dimension numerically the screening measures for the amplifier for a much higher amplification than is utilized in practice. Therefore, there may be provided at a desired point of the amplifier cascade one or more absorption members, which consists in the known fashion of a series resonance circuit,

i. e., the series connection of LR and C, and is tuned to the resonance peak of the chokes. By means of the series resistance there may be obtained a desired damping of this coupling. It is accomplished in this way that the resonance peak, which cannot be utilized in any case upon image operation, is flattened or eliminated entirely, and that the amplifying maximum is made to coincide with the maximum quality. It is preferable to apply this absorption element to the amplifier input or to the early stages of the amplifier. The application thereof to the amplifier output is less desirable.

The disadvantages in the use of conventional coupling means are substantially avoided by the use of a special transformer shown in the single figure. A transformer of this kind is to be used in the general circuit diagram of the apparatus at the input of the detector tube and again in the synchronisation apparatus. A transformer is given the desired characteristic by cutting the primary winding 2 into two halves 2 and 2', and feeding these half-windings at the centre from the anode potential source I to the amplifier tube I. If now a screening jacket having a lonitudinal slot 6 is placed over the centre of the primary winding 2, 2' divided in this fashion, no detrimental capacity will be introduced above the middle of the winding, and the same will add merely to the operating capacity of the anode, whereby the entire structure will certainly be detuned according to somewhat longer waves, but as a whole remains completely singlewave. It is possible in this fashion to couple the secondary coil 3 purely inductively with the primary coil.

I claim:

1. A wide band amplifier comprising a plurality of thermionic tubes each having input and output circuits, means for supplyin signals to the input circuit of one of the tubes. a load circuit connected as a part of the output circuit of the other of the tubes, a transformer for coupling the output circuit of the first of the tubes to the input circuit of the second of the tubes, said transformer comprising a pair of coaxially positioned and longitudinally spaced primary windings and a secondary winding, means for energizing the input circuit of the said second tube from the said secondary winding, means for energizing one of the transformer primary windings at the end terminal adjacent the other primary winding by output signals from the first of said amplifying tubes, means for applying operating voltages to the first of said amplifying tubes at the end terminal of the other of said primary windings adjacent the first named primary winding, and an electrical connection between the separated terminals of said co-axia1ly arranged primary windings.

2. A wide band amplifier comprising a plurality of thermionic tubes each having input and output circuits, means for supplying signals to the input circuit of one of the tubes, a load circuit connected as a part of the output circuit of the other of the tubes, a transformer comprising primary and secondary windings for coupling the output circuit of the first of the tubes to the input circuit of the second of the tubes, said transformer primary winding including a pair of co-axially positioned and longitudinally displaced inductive elements, connections from the secondary winding to the input circuit of the said second tube, means for energizing one of the transformer primary inductive elements at its terminal nearest the displaced primary winding by the output signals from the first of said amplifying tubes, means for applying operating voltages to the first of said amplifying tubes at the terminal of the other of said primary inductive elements nearest the other displaced primary winding, and an electrical connection between the remote terminals of said coaxially arranged primary inductive elements, an electrostatic shield means interposed between the transformer secondary winding and one of the primary inductive elements for reducing capacity effects whereby substantially only inductive coupling is obtained between .the output and in put circuits of the first and second of said amplifier tubes respectively.

3. An amplifier comprising a plurality of thermionic tubes each having an input and an output circuit, a transformer for coupling the output circuit of the first of said tubes to the input circuit of second of said tubes, said transformer including a plurality of longitudinally spaced and coaxially arranged primary'windings having inner and outer terminals, said inner terminal being substantially adjacent a conductive connection between the outer terminals of each of said primary windings, means for supplying signal output from the first of said tubes to the inner terminal of one of said primary windings, means for applying operating voltages for said first tube to the inner terminal of the other of said primary windings, said transformer windings also having a predetermined distributed capacity whereby the said complete primary winding becomes resonant at a frequency external to the pass-band frequency to be amplified, means for energizing the input circuit of the second of said tubes from the energy in the secondary winding of said transformer, and elec trostatic means interposed between one of the primary windings and the secondary winding for reducing capacity effects.

KURT SCHLESINGER. 

